A new photolithography based technique to mass produce microlens+fibre based integral field units (IFUs) for 2D spectroscopy

Long-slit astronomical spectroscopy has various limitations when dealing with optimum slit width, atmospheric dispersion, extended source spectroscopy, etc. to name a few. Most of these issues can be solved by the use of optical fibers as the light carrier from the telescope focal plane to the spectrograph. The approach is technically and scientifically flexible in terms of instrument modularity and target acquisition. Implementation of Integral Field Unit (IFU) provides a continuous sampling of extended objects and has a distinct advantage over the single fiber. Using a microlens array in front of the fibers improves the sky coverage by increasing the fill factor. Devasthal Optical Telescope Integral Field Spectrograph (DOTIFS) is a novel instrument being built by the Inter-University Centre for Astronomy and Astrophysics, Pune for the 3.6m Devasthal Optical Telescope (DOT) constructed by Aryabhatta Research Institute of Observational Sciences, Nainital. Each of the 16 DOTIFS IFUs consist of 12x12 spatial elements (spaxels) distributed in a hexagonal honeycomb structure covering 8.7"x7.8" in the sky. Each IFU is made by a photolithography technique to transfer the corresponding microlens array pattern to create a mask which holds the fibers at the focal plane end of an integral field unit. These masks are aligned with the microlens array and fibers are inserted before gluing and polishing. The fiber array can be positioned with a peak positioning error less than 5 μm from the desired position within a fiber array, compared to a requirement of 10 μm. The slit end is made by wire EDM cutting technology and fibers are placed with an accuracy of ~0.3 pixels compared to a 6.75 pixel center-to-center gap between two spectra on the detector. In this paper we provide details of deriving requirements and error budgets. The process of photolithography and the use of generated masks to create an IFU are also discussed. The technique allows very cost effective mass production of IFUs which are very accurately matched with the corresponding microlens array.